If you've ever observed a campfire, you've probably noticed smoke and floating bits of ash rising from the flames. These are purely a product of combustion: a rapid chemical process that produces heat and light. The smoke you see ascending out of an erupting volcano, on the other hand, consists mainly of tiny mineral particles formed by the explosive release of gases.
Plate tectonic activity in the Earth's crust forms deposits of molten rock (or magma). This magma contains trapped, pressurized gases. If the confining pressure of the magma decreases or gas pressure within increases, the contents simply blast through to the surface. For a complete look at this process, read How Volcanoes Work.
Volcanoes are like a spewing bottle of soda. When gas bubbles rush to escape your soda bottle, they wind up carrying at least some of the soda with them. In the case of volcanoes, escaping gases carry magma up into the air. The sheer violence of the explosion shreds the rising magma into tiny particles -- much like how a sneeze emits moisture in tiny droplets. These minuscule pieces of magma then solidify in the air, becoming volcanic ash. We call flowing surface magma lava.
Volcanic ash consists of tiny, jagged particles of rock, minerals and volcanic glass. These fragments range in size from 0.00004 inches (0.001 mm) to 0.08 inches (2 mm), roughly the diameter of a grain of rice. Volcanoes spew larger fragments than this, but scientists classify them as cinders, blocks or bombs. Volcanic ash is hard and, due to each particle's jagged form, abrasive to the touch. The exact mineral makeup of the ash depends on what minerals were present in the magma.
Once volcanic ash is airborne, three factors determine how far it will travel before falling back to the Earth:
1. Particle size: The larger a particle of volcanic ash is, the closer it will fall to the volcano. Likewise, the smaller the particle, the farther winds will carry it.
2. Wind speed and direction: Airborne clouds of ash will travel in whatever direction the winds carry them and at whatever speed the wind travels. A strong, constant wind will carry volcanic ash away in a relatively straight line. Rotating storm-type winds, however, can distribute volcanic ash in many different directions.
3. Eruption type: There are several different kinds of volcanic eruptions, and their severity plays into both of the above factors. Eruption type determines the quantity of ash, the size of the ash particles, as well as how high into the atmosphere they travel. Particularly powerful eruptions can blast particles into the very upper levels of the planet's atmosphere.
Some of this volcanic ash mills around the air, joining other dust particles in the atmosphere as condensation nuclei, which water vapor condenses around to form clouds. Some violent eruptions can even add enough volcanic ash cloud cover to the upper atmosphere to drop global temperature by several degrees while the particles spread slowly over the entire planet. For instance, the 1883 eruption of Krakatoa lowered global temperatures by 2.2 degrees F (1.2 degrees C) for a year [source: The Independent].
For the most part, however, what comes up must come down. On the next page, we'll follow the ash back to Earth.